"Onium salts" are salts having the general formula EQU R.sub.a Q.sup.+ An.sup.-
where An.sup.- is a salt forming anion, R is aryl or alkyl, and Q is:
(1) a halogen and a=2; PA1 (2) a group VIA element and a=3; or PA1 (3) a group VA element and a=4. PA1 Q is I, S or P; PA1 R.sub.1, R.sub.2 and R.sub.3 are independently aromatic groups having 6-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; or aliphatic groups having 1-12 (preferably 1-4) carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; or aralkyl groups having 7-18 (preferably 7-10) carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or a alkylamino groups; PA1 M.sup.+ is a cat ionic organic radical; PA1 A is a divalent radical selected from the group of hindered alkylene groups having 4-12 carbon atoms; aromatic groups having 6-18 (preferably 12-18) carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; and aralkyl groups having 7-24 (preferably 7-18) carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; PA1 B is a divalent aromatic sensitizer which absorbs radiation having a wavelength longer than 300 nm and is capable of transferring an electron to Q, PA1 x.sup.- and X'.sup.- are anionic groups; and PA1 Q=I; PA1 a=1 PA1 b and c=0 PA1 Q=S; PA1 c=0 PA1 a and b=0, 1, or 2 such that a+b=2 when R.sub.1 and R.sub.2 are monovalent radicals; PA1 a+b=1 when R.sub.1 or R.sub.2 is a divalent radical; PA1 Q=p; PA1 a, b and c=0, 1, 2 or 3 such that a+b+c=3 when R.sub.1, R.sub.2 and R.sub.3 are monovalent radicals; PA1 a+b+c=2 when one of R.sub.1, R.sub.2 or R.sub.3 is a divalent radical; and
The most common examples of these salts are the diaryl iodonium salts, the triaryl sulfonium salts and the tetraaryl phosphonium salts.
Onium salts are known to function as photoactivators for cationic polymerization and acid-degradation reactions, as disclosed, for example, in U.S. Pat. Nos. 4,136,101 and 4,603,101. Most of the onium cations absorb in the deep UV region, i.e., less than 300 nm, and therefore are only useful as initiators when activated by such short wavelength radiation. It is possible to alter the spectral response of the onium salt by the addition of a sensitizer, for example perylene, as disclosed in U.S. Pat. No. 4,603,101. The sensitizer absorbs radiation at a wavelength longer than that where the onium group absorbs and after absorption transfers an electron to the onium moiety. The effectiveness of the sensitizer depends upon is ability to interact directly with the onium salt. However, in most systems both the onium salt and the sensitizer are present in small amounts and the encounter probability of two low-concentration species is generally quite small.
To improve the effectiveness of sensitization, the onium salt must be kept in close spatial proximity to the sensitizer. One approach to accomplish this is to covalently tie the sensitizer to the central onium ion through one of the R groups. Such an approach has been disclosed by Pappas and Tilly [M. S. Tilly, Doctoral Dissertation, North Dakota State University, Fargo, N. Dak., October 1988] and in U.S. Pat. No. 5,047,568. These references disclose compounds having the general formula EQU Ar--(--R.sub.1 --)--S.sup.+ (R.sub.2) (R.sub.3) A.sup.-
where Ar is a sensitizing aromatic group and A.sup.- is an anion. In the compound disclosed in Pappas and Tilly, Ar is 9-anthryl, R.sub.1 is propyl, and R.sub.2 and R.sub.3 are phenyl.
A second approach to accomplish the spacial linkage of the sensitizer is to put the sensitizer in the anion so that the onium moiety and the sensitizer are held together by coulombic attraction. Such an approach has been disclosed in U.S. Pat. No. 4,772,530. In this reference, diphenyl iodonium, diphenylmethyl sulfonium, and triphenylmethyl phosphonium cations are paired with an anionic sensitizer which is a derivative of Rose Bengal.
In both of these prior art approaches, there is considerable flexibility in the linkage between the onium moiety and the sensitizer which allows the two moieties to assume a number of configurations. However, generally only one configuration will result in electron transfer to effect sensitization of the onium salt. Thus, the efficiency of these compounds is compromised by the flexibility and multiple configuration possibilities. In general, the quantum yields of reactions using these compounds are significantly less than 1.0.
There exists, therefore, a need for a sensitized onium salt in which the onium moiety is linked to the sensitizing moiety in such a way as to obtain more effective sensitization of the onium moiety.